Hermetic seals usually are required for optoelectronic devices to make the device impervious to external influences, such as moisture. Low melting solders are desirable for sealing because the low melting temperatures will reduce the thermal induced warpage in these devices. Typical examples of low-melting solders used to form hermetic seals include indium (Tmelt=156° C.), 52In-48Sn (Tmelt=118° C.), 58Bi-42Sn (Tmelt=138° C.), and 63Sn-37Pb (Tmelt=183° C.).
The hermetic seal is produced when the base metal, Au or Cu for example, dissolves into a molten metal, such as Sn or In and their alloys, and the active constituent in the solder combines with the base metal to form an intermetallic compound. The relative amount of base metal that goes into the solution is related to its solubility in the solder, and the intermetallic compound that forms depends on the solubility of the active element in the base metal. This process depends on the length of time that the intermetallic phase temperature remains above the liquidus temperature of the solder.
It is known that Au and Cu, when compared with other noble metals such as Ag, Pt, and Pd, quickly dissolve into molten Sn, In, and their alloys, and rapidly react and form intermetallic compounds with Sn, In, and their alloys. When a molten layer of Sn or In is sandwiched between layers of Au or Cu, the whole layer of molten metal can be consumed by formation of a high melting intermetallic compound or a compound with boundary Au or Cu layers during the usual soldering process. When the molten layer of an Sn or In eutectic alloy is sandwiched between Au or Cu layers, then Au or Cu usually reacts with one element in the Sn or In alloy. This causes the Sn or In alloy to move away from its eutectic composition, and this leads to a higher melting temperature for the seal.
In the past, the Au or Cu used to make these seals has not been very thick, usually about 0.1 μm, and the melting temperature of the resulting seal has not been high enough to make subsequent step soldering, which is frequently used in manufacturing situations, easy to perform. Furthermore, there are problems with creep resistance, dimensional stability, and optical performance of the seal when placed in the optoelectronic device.